The present invention relates to a process for preparing a high quality carbon fiber having a high strength by using an acrylic fiber as the precursor.
Carbon fibers have heretofore been widely used as reinforcing fibers of various composite materials due to their superior mechanical strength characteristics such as excellent specific strength and specific elasticity. In production of a high-grade carbon fiber having an especially high strength among others, an acrylic fiber is usually used as the precursor in a generally adopted process comprising the steps of heating an acrylic fiber in an oxidizing atmosphere at a temperature of about 200.degree. to 400.degree. C. to oxidize the same for conversion thereof into an oxidized fiber (flame-retardant fiber), and heating the resulting fiber in an inert atmosphere of nitrogen, helium, argon, or the like at a temperature of about 400.degree. C. or higher to carbonize the same. Thus, acrylic fiber precursor is subjected to a very severe thermal treatment during the course of conversion thereof into a carbon fiber.
More specifically, it is believed that acrylic fiber precursor is converted into a fiber having a thermally stabilized molecular structure as a result of cyclization and crosslinking reactions of polymer chains constituting the acrylic fiber in the above-mentioned oxidizing step, while at the same time adherent substances included in the fiber, such as a oiling agent, are evaporated and thermally decomposed, followed by polymerization of the resulting thermal decomposition product into a tar-like substance. Not only fusion of filaments constituting a fiber bundle among each other but also surface and inside defects of the filaments themselves are caused by the reactions of the polymer chains constituting the fiber and the thermal decomposition of the adherent matters included in the fiber, such as an oiling agent. Such fusion and defects drastically deteriorate the physical properties of the carbon fiber. Thus, prevention or suppression of such fusion and defects is very important in the industrial production of carbon fibers.
Against the above-mentioned problem peculiar to the production of a carbon fiber, a number of proposals have been made of the use of an organosilicon compound as an oiling agent for an acrylic fiber with a view to suppressing the fusion of filaments among each other in the process for production of a carbon fiber, particularly in the oxidizing step.
However, the use of such an organosilicon oil involves a problem of worsening of a working environment and causing pollution since the oligomer and terminal molecular chain of the organosilicon oil are decomposed and evaporated at high temperature and further decomposed into silicon dioxide in the process for production of a carbon fiber, with the result that a gas exhaust unit is clogged, while dust heaps up. Further, since the organosilicon does not always satisfy the performance required of a processing or finishing oil for an acrylic fiber by itself, it is not usually used alone but in combination with other processing or finishing oil. In this respect, however, the effect of suppressing fusion of the filaments among each other by the organosilicon oil is ruined through it depends on the combined oil, or the defects of the carbon fiber are caused by a tar-like substance converted from the combined oil in the above-mentioned oxidizing and carbonizing steps, thus presenting a problem.